Mirror modifier, but not quite. Is there a phantom modifier?
We'll start with this mesh (I extruded a face so it isn't symmetric):



Go to the object tab and add the object to a group:



Now, create an empty. Go to the object tab and set the duplication to the group you just created:



Now, to mirror the object, simply make the x-scale of the empty negative. It will become flipped. Since Blender is smart enough, the normals will be alright.



Now, you have a flipped copy of the object that will change if you change the original. You can repeat this if you need more copies, and you can position them as you wish: